Commonly assigned U.S. Patent Publication No. 2006/0220851, the disclosure which is hereby incorporated by reference in its entirety, discloses a system and method for tracking containers in grounded marine terminal operations. The system tracks cargo containers and vehicles contained within a terminal and includes a tag transmitter positioned on container handling equipment, containers, cranes and other marine terminal equipment. In that disclosure, the tag transmits a wireless RP signal based on an event affecting the location of a container handled by the container handling equipment. A plurality of spaced apart access points are positioned at known locations within the terminal that receive the wireless RF signals from the tag transmitter. A processor is operatively connected to the locating access points for geolocating the tag transmitter and determining the container location at the time the event occurs such as by using time difference-of-arrival measurements. GPS sensors can be located on equipment masts or other locations to enhance the system.
Some location systems are now using a differential global positioning system (DGPS) as an enhancement to global positioning systems in which one or a network of fixed ground based reference stations (receivers) broadcast a difference between the positions indicated by a satellite system and known fixed positions. The stations can broadcast differences between the measured satellite pseudoranges and actual or internally computed pseudoranges. It is possible to use an “inverse” DGPS enhancement. The term “inverse” could apply to correcting position data at a control center, dispatcher site or other processor. These systems provide better accuracy than using GPS systems alone because several sources of error can occur with GPS alone. These sources of error include ionospheric effects and errors in the satellite position ephemeris data and clock drift on the satellites. The DGPS correction signal can correct for these effects and reduce error significantly depending on the amount of data that is sent, and some accuracies can be obtained that are less than 20 cm and in some instances, under better conditions, accuracies can be achieved of under 10 cm.